This invention pertains to a resin composite material having a metal foil on top of a resin substrate, and a method for forming such resin composite material. This invention also pertains to a metal foil attached to a carrier resin used in this method for forming a resin composite material.
Metal-resin composite materials, beginning with copper-clad laminate boards, are used in a wide range of industrial fields such as electronic parts and mechanical parts. Especially with the rapid development of the electronics industry in recent years, copper-clad laminate boards have come to be used in a wide range of fields such as flexible printed circuit boards (FPC), film carriers for TAB (Tape Automated Bonding), multilayer circuit boards, or COF (Chip On FPC) in electronic devices such as hard disk drives, liquid crystal display elements (LCD), portable telephones, game machines, automobiles, cameras, or audio equipment. Various resins are used for the resin substrate in copper-clad laminate boards depending on factors such as use conditions and working characteristics, but polyamide resin is widely used as a resin substrate from the standpoint of superior properties such as heat resistance, flame resistance, mechanical strength, and electrical characteristics.
A number of methods are known for placing copper foil on resin substrates, beginning with polyimide resin substrates, and fabricating copper-clad laminate boards. One method is the method of forming copper foil directly on top of a resin substrate by applying electroless copper plating to a resin substrate, then applying electrolytic copper plating as required. Other methods using copper foil formed first are the laminating method of adhering copper foil to a resin substrate either with or without using an adhesive, then hot-fusing the copper foil to the resin substrate, and the casting method of forming a film by pouring molten resin over copper foil. Still another method is the sputtering method of sputtering copper onto a resin substrate in a vacuum, then obtaining the desired thickness of copper foil by electroless copper plating with the sputtered copper as the nucleus. Still another method, disclosed in Japan Unexamined Patent No. 8-209354, is the method of introducing acidic groups into a resin substrate and introducing metal ions, then reducing this to form a metal coating, followed by applying electroless or electrolytic copper plating as required.
In the method of forming copper foil directly on top of a resin substrate by applying electroless copper plating to a resin substrate, then applying electrolytic copper plating as required, however, palladium is used as a catalyst during electroless copper plating, and therefore, palladium must be removed during pattern formation. In addition, using particular resins, beginning with polyimide, can make it difficult to improve adhesion between the copper deposited electrolyticly and the resin substrate. Furthermore, even with resins capable of improving adhesion between electrolytic deposited copper and the resin substrate, such as epoxy resins, the surface of the resin substrate is subject to injury by etching which is done to improve adhesion.
In the laminating and casting methods using copper foil formed first, handling concerns limit the thickness of the copper foil used to a minimum of 8 μm. When a fine-pattern substrate is demanded for reasons such as developing the high-performance electronic devices of recent years, thinner copper foil is demanded in copper-clad laminate boards. In addition, when a copper-clad laminate board having 8 μm or less copper foil thickness is required, copper foil must be made thinner by a means such as etching after forming a conventional copper-resin composite material. Etching copper foil, however, has the problem that it is difficult to etch copper foil evenly, and copper foil becomes uneven in thickness.
The sputtering method has the problems of the large size of its apparatus, high costs, and lower productivity compared to other methods.
Furthermore, although the method of introducing acidic groups into a resin substrate and introducing metal ions, then forming a metal coating by reducing the metal ions, followed by applying electroless or electrolytic copper plating as required has the advantages of superior adhesion between the resin substrate and metal foil as well as being able to form a metal foil of a desired thickness uniformly, it has the problem that the surface of the resin substrate is subject to injury when introducing acidic groups to the resin substrate. In addition, it can be difficult to introduce acidic groups depending on the type of the resin used. Moreover, although the source cited earlier discloses that metal-resin composite materials formed by such a method have superior adhesion between the metal and the resin substrate, it is difficult for persons skilled in the art to transfer this superior adhesion to metal foils formed by such a method.